A plasma welding method, because of its high energy density, attains higher welding efficiency than what is allowed for a TIG welding method, and the welding quality is not any lower than that of the TIG welding. A plasma powder welding method has been widely used as a bead-welding method and a method for improving a surface quality of a work piece.
In a conventional method the powder used for the plasma powder welding includes a component which is different from a component of a work piece for securing a high wear-resistance, for instance. (Reference, as an example, is made to Japanese Patent Examined Publication No. H11-277246, pages 2 to 4 and FIG. 1). A torch is set at a large distance from the work piece, and focal length of jet of the powder is set shorter than the distance of the torch from the work piece. (Refer to Japanese Patent Examined Publication No. H8-300157 (pages 2 to 3, FIGS. 1 and 2), as an example).
FIG. 4 illustrates a relationship between a tip portion of a chip of a welding torch and a work piece, in the conventional plasma powder welding method. The plasma powder welding device includes chip 101, plasma nozzle 102, powder nozzle 103, electrode 104, and work piece 105. In FIG. 4, illustrated also are a distance ‘h’ of the torch from the work piece 105, a diameter of an imaginary circle ‘P’ configured by openings of the powder nozzles, an angle ‘a’ formed by intersecting axes of the powder nozzles, and a focal length ‘d’ of the intersection from the tip of chip 101.
Operation of the plasma powder welding method thus constituted will be explained next. A plasma arc is generated between electrode 104 and work piece 105 through plasma nozzle 102. Meanwhile a powder is ejected with a carrier gas through an opening portion of powder nozzle 103. The ejected powder is heated by the plasma arc and transferred to a surface of work piece 105. The distance ‘h’ of the torch from the work piece is set long in a range of 10 mm to 20 mm for preventing over melting of the work piece, and the focal length ‘d’, which is determined by a combination of the diameter of the imaginary circle ‘P’ and the angle at the intersection ‘a’, is set in a range of 50 to 60% of the distance ‘h’. Thus, the ejected powder is first exposed to the plasma arc, and then dissolved by the heat to be deposited on work piece 105. With this arrangement, the powder is selectively dissolved and over melting of the work piece is controlled, thereby a component of the powder is prevented from being diluted with a component of the work piece when deposited on the surface of the work piece.
As described above, the conventional plasma powder welding method aims to deposit the powder having a different component from that of the work piece to the surface of the work piece, for improving the surface quality. For this purpose, melting of the work piece is controlled to a minimum so that the powder may not be diluted with a component of the work piece thereby losing its original composition, or being compounded with the work piece component to produce a deleterious substance. As a consequence, the conventional welding method is not appropriate for joining work pieces which require sufficient melting of the pieces.